1. Field of the Invention
The present invention relates generally to hydraulic machinery and more specifically relates to hydraulic motor/pumps whose mechanical advantage can be varied during their operation.
2. Background of the Prior Art
Since the development of the water wheel and inclined screw pump in classical times, humanity has used hydraulic machinery to derive mechanical motion from flowing fluids and vice versa.
a. Specific Pumps
Hydraulic machines, for example the Pappenhein, Cochrane, Cary, Pattison, Ramelli, Emory, Heppel, Knott, Repsol, and/or Holly, rotary pumps use eccentric interlocking stators or gears to convert rotary mechanical energy into fluid pressure. The Quimby screw pump is illustrative of a class of hydraulic pumps that use screw threads in a cylinder to move viscous fluids. Other examples of higher pressure rotary pumps include those developed by Gould, Greindl, Mellory and Hasafan.
The above pumps generally have two mated gears or lobes closely fitting within a casing. One gear is the driver, the other gear, the follower, is driven by the driver. Many adaptations have been tried to obtain an efficient rotary pump.
The Gerotor pump uses an inner gear that is keyed to and rotates with, the driving shaft; an outer gear of internal type is driven by the inner gear and is free to rotate with a snug fit in a recess in one end of the housing. The teeth of the two gears are specially shaped so that the tops of all teeth of the inner gear are always in sliding contact with the teeth of the outer gear.
The Vickers vane pump is a constant discharge pump in which radial vanes produce the pumping action. The vanes are free to slide in and out of a rotating hub and so maintain contact with an outer ring. Oilways from the high pressure discharge of the pump to the spaces behind the vanes assure that this contact is maintained.
Unfortunately, rotary pumps are less efficient than piston pumps. Piston pumps include valve plate axial piston pumps, in which pistons are driven by a non-revolving wobble plate, and bent axis valve plate axial piston pump, wherein the angle between two sections of the pump housing, which may be adjusted by hand or servo control, determines piston travel.
The Hele-Shaw radial piston pump converts the rotary motion of an eccentric shaft into motion of pistons pumping fluid.
The centrifugal pump and its functional converse the turbine motor use centrifugal and reaction force, respectively, to accomplish their purposes.
b. General Discussion
Rotary Pumps are of the positive-displacement type, usually valveless, simple, compact, light in weight, and low in first cost. They are built in capacities from a fraction of a gallon (as in domestic oil burners and refrigerators) to 5,000 gpm and above, as in marine cargo service. Though used for pressure up to 1,000 psi, their particular field is for pressures of 25 to 500 psi. Before the development of the modern centrifugal pump, large rotary pumps of lobe type were used for low-head irrigation projects in capacities as large as 35,000 gpm and showed mechanical efficiencies of 80 to 85 percent.
Rotary pumps require the maintenance of very close clearances between rubbing surfaces for their continued volumetric efficiency. No satisfactory method of packing the moving surfaces to compensate for wear has been developed; consequently, although some rotary pumps are used successfully for clean water, their great field of application is in pumping oils or other liquids having lubricating value and sufficient viscosity to prevent excessive leakage. Rotary pumps are being used in the oil industry in increasing volume. They are also used for liquids of high viscosities.
Pigott (Oil Gas Jour., May 10, 1934) classifies rotary pumps in the following seven groups: (1) vane type, (a) sliding vanes, (b) swinging vanes; (2) oscillating-piston or eccentric type; (3) gear type, (a) lobar, two and three teeth, (b) special-contours teeth, (c) spur gear, (d) helical and herringbone gear, (e) internal gear with two-teeth differences or with one-tooth difference; (4) screw type; (5) radial plunger type; (6) swash-plate type; and (7) miscellaneous.
Rotary pumps up to 100 psi may be considered low pressure, from 100 to 500 psi moderate pressure, and above 500 psi high pressure; fractional to 50 gpm are small-volume pumps, 50 to 500 gpm moderate-volume, and above 500 gpm large-volume.
Vane Pumps. Leakage in vane-type pumps occurs across the tips and sides of the vanes. Since the vane tips cannot be made to fit the bore of the housing in all positions, there is line contact and low resistance to leakage. Wear is serious at the higher speeds unless the vanes are restrained against centrifugal forces. Increasing the number of vanes materially decreases leakage, but increases cost and complexity.
Guided-vane Type Pumps. A single rotor revolves in a case. The pumping element consists of multiple blades sliding in and out of slots in the rotor. Impeller and case are eccentric. Centrifugal force or pressure maintains the outer end of the blades in contact with the casing bore. The blades are made of hardened steel, bronze, or bakelite. This type of pump is useful for small and moderate capacities and low pressure. Rapid wear on the points of the sliding blades and in the casing occurs where speed is high or where the liquid pumped has a low lubricating value. In some constructions, the blades are made with end trunnions operating in grooves in the side plate.
Swinging-vane Type Pumps. This type of pump has vanes that are hinged or articulated. The hinge joints are subjected to wear, and the comparatively small number of vanes or blades possible with this construction give a less satisfactory seal than do the multiple blades in the sliding-vane type. Swinging-vane pumps are used for moderate volume, for low pressure and vacuum, and for low speeds.
Eccentric-piston Pumps. Many pumps of this type are in service. The contact between the strap and the body approximates single-line contact. Leakage, therefore, becomes excessive as wear progresses. This type of pump is useful for small and medium capacities, low pressure, and limited speed.
Radial-plunger and Swash-plate Pumps. The rotation of the body carrying the plungers connects each plunger flow periodically to the suction port on the plunger's suction stroke and to the discharge port on its discharge stroke. These can be adapted for variable capacity by varying the eccentricity between the plunger-carrying body and the ring that drives the plungers; or by varying the angle between the drive shaft and the plunger-carrying body. The actual machines are complicated.
Lobar Pumps. Lobar pumps are suitable for medium and large capacities and low pressures. As in the oscillating-piston type, there is line contact between the impeller and the body, and leakage is excessive at higher pressures. The impellers are not self-actuating. Such pumps, therefore, must be built with external pilot gears capable of transmitting half the power utilized from the driving to the driven shaft.
Gear Pumps. These pumps are of the two-shaft type and cover a wide variety of constructions. They are used for practically all capacities and pressures. In many types, the impeller gears are self-actuating, requiring no pilot gears. The simplest form uses spur gears. The large number of teeth in contact with the casing minimizes leakages around the periphery. The utility of the straight spur-gear type is limited by trapping of liquid, which occurs on the discharge side at the point of gear intermesh, resulting in noisy operation and low mechanical efficiency, particularly at high rotative speed. Discharge pockets in the side plates may be provided to reduce the effects of trapping. Impellers in other pumps of this type are of single-helical or double-helical construction with angles from 15 to 30 degrees or more. With gears of single-helical type on higher pressure, considerable end thrust of the impeller gears on the pump side plates results. Either helical or herringbone gear construction largely eliminates the effects of trapping but introduces leakage losses between the teeth at the meshing point unless the teeth are cut without root clearance.
Internal-gear Pumps. "One-tooth difference." In pumps of this type, an impeller mounted eccentrically with the body actuates an internal gear rotating in the body or in bearings carried in the end plates. Flow is practically continuous and without reversals. High rotative speeds may be used. In such pumps, leakage occurs around the periphery of the ring gear, over the tips of the gear teeth at open mesh, and through the contact line at full mesh. This type is particularly adaptable for high pressures and high speeds, for oils with lubricating value and considerable viscosity.
"Two-teeth difference." In this construction an abutment on one side plate is used to fill the clearance between the external and internal gear. Such construction reduces leakage, but involves the use of an overhung internal gear that restricts the pump's application to small and medium capacity and pressure.
Screw Pumps. In this type of pump, a long single helical impeller of small diameter and special form actuates one or more idler impellers contained in a casing so as to displace the liquid pumped axially. Multiple surface, rather than line contacts, between screws and case minimizes leakage. This construction permits operation at very high speed. Where, right- and left-hand helices are used, the pumping load is balanced and thrust is eliminated. No shaft bearings or timing gears are required owing to the form of the impellers. Wear of rotating elements may be rapid with liquids of low lubricating value.
Double-screw Pumps. Double-screw pump construction incorporates right- and left-hand intermeshing helices on parallel shafts with timing gears. These pumps have been extensively used for medium and large capacities and moderate to high pressures. There is some leakage axially at the impeller contact. Impellers are carried in bearings so that wear on impellers and casing is reduced. Flow is practically continuous.
The mechanical efficiency of the better types of rotary pumps when handling oils or other liquids with lubricating value is good.
Aside from leakage, wear problems and strictly limited ranges of capacities and pressures, the conventional hydraulic pumps described above are generally not good hydraulic motors and vice versa.
A simple reciprocating piston connected to a flywheel doesn't leak, doesn't wear when pumping non-lubricating fluids and is both a good hydraulic pump and an efficient hydraulic motor. This arrangement's greatest defect is that its mechanical advantage is constant, or at least cannot be altered while in operation. The result of this limitation is that the speed and torque of such a motor is a function of fluid flow and pressure. Used as a pump, its delivery is a function of flywheel speed and its output pressure is a function of input torque for a given constant load.
In the past these limitations have been avoided by operating reciprocating machines at variable speeds, by bypassing some of the pumped fluid around the pump at constant speed, or by intermittently loading and unloading the pump. The size of the piston may also be changed, but not while the system is in operation. All of these expedients either require that the system be stopped or that it lose efficiency.